in general, any of nearly 200 species of tailed primate, with the exception of lemurs (lemur), tarsiers (tarsier), and lorises (loris). The presence of a tail (even if only a tiny nub), along with their narrow-chested bodies and other features of the skeleton, distinguishes monkeys from apes (ape). Most monkeys have a short, relatively flat face without great prominence of the muzzle, although baboons (baboon) and mandrills (mandrill) are notable exceptions. The vast majority of species live in tropical forests (tropical rainforest), where they move on all four limbs. All but the durukuli of tropical Central and South America are active during the day, moving frequently in bands as they search for vegetation, birds' eggs, smaller animals, and insects to eat. Monkeys are capable of sitting upright, and, consequently, their hands are freed for many manipulative tasks. Except for a few Old World forms, monkeys are predominantly arboreal, leaping from limb to limb in their travels among the trees. Their hands and feet are both used for grasping and typically have five digits, the thumb and big toe being divergent from the others. Commonly, the digits have flattened nails (nail), but the marmosets (marmoset) have claws on all digits except the big toe, which bears a nail. On the ground, monkeys walk with the entire sole of the foot touching the ground but with the palm of the hand raised. They almost never walk on two legs (bipedally) and can stand erect for only short periods, if at all.

Monkeys have large brains and are known for their inquisitiveness and intelligence. Brain development, combined with the freeing of the hands and well-developed vision, allows them a great latitude of activity. Most are good at solving complex problems and learning from experience, but they do not quite reach the cognitive levels of great apes. Some, especially the capuchins (capuchin monkey) (genus Cebus), spontaneously use objects as tools (e.g., stones to crack nuts). Others, such as baboons, readily learn to use sticks to obtain food. However, in strong contrast to the great apes (gorillas, chimpanzees, and orangutans), most monkeys do not appear to be very good at learning from each others' experience—individuals more or less have to learn new behaviours for themselves. A significant exception is the Japanese macaque (Macaca fuscata). In field experiments, these monkeys were introduced to new foods such as sweet potatoes and candies wrapped in paper. Once a few individuals had solved the problems of getting at the new foods, their innovations gradually spread throughout entire troops. These experiments have had implications in redefining cultural behaviour.

Monkeys are highly social animals, and almost all live in troops consisting of several females with young and either a single male (as in hamadryas baboons, mandrills, most guenons (guenon), and most langurs (langur)) or several males (as in savannah baboons and macaques). Usually, but not universally, the females stay in the troop in which they were born and are thus closely related to each other. Males join new troops on maturity, and so they are unrelated to each other and somewhat antagonistic. Like humans and apes, female monkeys nurse their young and have a menstrual cycle, albeit less copious. In some species, sexual activity is strictly confined to the period around ovulation ( estrus); in others, there appears to be little or no restriction. Some species breed all year round; others have a period several months long during which they experience no sexual cycles (anestrus).

Old World monkeys live throughout Africa, on the Red Sea coast of Arabia, and in Asia from Afghanistan to Japan and southeast to the islands of the Philippines, Celebes, Bacan, and Timor. Some Old World monkeys have been successfully naturalized in Gibraltar, France, Mauritius, Belau, and a few islands of the West Indies. Old World monkeys include many that are often seen in zoos, especially the beautifully coloured African guenons (e.g., mona (mona monkey), diana (diana monkey), white-nosed, green, vervet, and grivet monkeys), colobus, mangabeys (mangabey), and the chiefly Asiatic macaques (macaque). The macaques include the Barbary “ape” (Barbary macaque) of North Africa and the Rock of Gibraltar—the only macaque outside Asia and the only wild monkey inhabiting any part of Europe today—and the rhesus monkey of the Indian subcontinent, which has been used considerably in medical research. The graceful langurs (langur) include the hanuman, or sacred monkey, also of southern Asia. Among the more unusual monkeys are the large and strikingly coloured African drills (drill) and mandrills (mandrill), the proboscis monkey of Borneo, and the rare and bizarre snub-nosed monkeys (snub-nosed monkey) of China and Vietnam. The Old World monkeys are divided into two subfamilies: Cercopithecinae and Colobinae. The cercopithecines have cheek pouches, in which they store food; these include baboons, macaques, guenons, and their relatives. The colobines lack cheek pouches but have complicated three- or four-chambered stomachs, where bacterial fermentation of cellulose and hemicellulose occurs and thereby enriches the nutrient content of their diet, which consists partially of leaves and seeds. Colobines include colobus monkeys, langurs, and their relatives.

103 or more species in 21 genera from Africa and Asia. The number of species stated within a given genus may vary, depending on the taxonomic criteria used.

7 African species.

10 or more Southeast Asian species.

94 or more species in 5 families from tropical Central and South America. The number of species stated within a given genus may vary, depending on the taxonomic criteria used. Formerly, only two families were recognized within the group: Callitrichidae (marmosets and tamarins) and Cebidae (all others, including capuchins, titis, squirrel monkeys, and howler monkeys). Molecular evidence, together with reassessments of morphological evidence, now indicates that marmosets are more related to the capuchins, with spider monkeys and their relatives being more divergent. Recent classifications, therefore, tend to recognize additional families: Atelidae (spider monkeys and their relatives), Pitheciidae (sakis and uakaris), and Aotidae (durukulis); Callitrichidae and Aotidae are sometimes lumped into the Cebidae.